The present invention relates to a copier, a printer, a facsimile and an image forming apparatus employing an electrophotographic method and having the functions mentioned above, and specifically relates to a color image forming apparatus that is provided with an intermediate transfer member on which a plurality of unicolor images are overlapped on each other so as to form a full color toner image.
Well known has been a color image forming apparatus that employs the electrophotographic method in which a toner image formed on an image bearing member serving as a photoreceptor element is transferred onto the intermediate transfer member (a primary transferring operation), and then, the toner image transferred onto the intermediate transfer member is further transferred onto a transfer material (a secondary transferring operation). In such a color image forming apparatus, unicolor toner images sequentially formed on a plurality of image bearing members and charged in a predetermined polarity are transferred onto the intermediate transfer member by employing electrostatic actions in such a manner that the unicolor toner images are superimposed on each other, and then, a full color toner image formed on the intermediate transfer member is further transferred onto the transfer material by employing an electrostatic action.
Since it is possible for the color image forming apparatus employing the intermediate transfer member to superimpose the unicolor toner images formed on a single or a plurality of image bearing member(s) on the intermediate transfer member, such a color image forming apparatus has been widely applied as a color image forming apparatus to form full color images on the transfer material. In this type of color image forming apparatus, the unicolor toner images formed on a single or a plurality of image bearing member(s) are transferred onto the intermediate transfer member in such a manner that the unicolor toner images overlap each other, and then, the overlapping toner image formed on the intermediate transfer member is further transferred onto the transfer material by employing the electrostatic action.
Since an electrostatic charge amount per one toner particle is substantially uniform among plural toner particles, an electric potential of the toner layer residing on the intermediate transfer member varies depending on an amount of toner attached within a predetermined area. Accordingly, in this color image forming apparatus, within the overall toner image formed on the intermediate transfer member, an electrostatic charge potential of a partial area on which plural unicolor toner images overlap each other is greater than that of other partial areas on which only a single unicolor toner image exists. Further, for instance, when the overall toner image formed on the intermediate transfer member includes both a solid color area and a halftone color area, an electrostatic charge potential of the solid color area is greater than that of the halftone color area.
Further, variation of the electrostatic charge potential within the overall toner image after passing through a primary transferring section, at which the unicolor toner image is transferred from the image bearing member to the intermediate transfer member, sometimes would occur due to environmental factors.
When the electric potential of the toner image residing on the intermediate transfer member widely varies as mentioned above, various areas whose transferring characteristics are different from each other, coexist in the same toner image. Therefore, attempting to transfer such image areas, whose transferring characteristics differ from each other, onto the transfer material under the same transferring condition, various kinds of image defects are liable to occur at the time of the secondary transferring operation from the intermediate transfer member to the transfer material.
In recent years, the color imaging trend has proliferated in the field of copiers, printers, facsimiles and compound image forming apparatus combining the functions of the above-mentioned apparatuses, and, as for the transferring process, the demands for producing high-quality images have been getting larger than ever, due to the progressing trend of employing a polymer toner and a micro particle toner. In addition, the trend of increased production of the image forming apparatuses has also progressed. To obtain excellent images while meeting such trends, it is necessary to compensate for the electric potential of the toner layer residing on the intermediate transfer member, which is liable to vary depending on a number of primary transferring operations and the environmental factors, so as to make it uniform over the toner layer and to improve the secondary transferring efficiency.
Patent Document 1 (Tokkaihei 10-274892, Japanese Non-Examined Patent Publication) sets forth an apparatus that is provided with a pre-secondary transfer charging section for making an electrostatic charge potential of a toner layer residing on an intermediate transfer member uniform by applying a bias voltage, having a polarity same as that of a toner, onto a toner image, before transferring the toner image onto a transfer material.
Patent Document 2 (Tokkaihei 11-143255, Japanese Non-Examined Patent Publication) sets forth an apparatus in which a toner layer residing on an intermediate transfer member is made uniform by applying a bias voltage, having a polarity the same as that of a toner, onto a toner image, and at the same time, a potential difference controlling section controls a direct current electric power source (hereinafter, referred to as a DC power source, for simplicity) of a pre-secondary transfer charging section as well as another DC power source of a secondary transferring section, so that a potential difference between an electric potential of the toner layer, which is made uniform by the pre-secondary transfer charging section, and another electric potential of the secondary transferring section is kept substantially constant.
The color image forming apparatus set forth in Patent Documents 1 and 2 (Tokkaihei 10-274892 and Tokkaihei 11-143255, both being Japanese Non-Examined Patent Publication) compensates for an electrostatic charge amount of toner upstream side of the secondary transferring section by employing a scorotron charger. In other words, this is a technology for evening an electrostatic charge amount of toner, primarily transferred onto the intermediate transfer member, by employing a corona discharging phenomenon, such as an AC discharging action, a DC discharging action, etc.
On the other hand, in order to prevent density unevenness caused by a transferring charge shortage occurring at the time when a potential of the toner layer is high due to an excessive amount of attached toner, and to prevent occurrence of discharging action when increasing a transferring charge amount, it could be considered that the pre-secondary transfer charge eliminating section, having a scorotron electrode, is disposed upstream from the secondary transferring section so as to conduct a discharging operation of the toner image on the intermediate transfer member.
A scorotron charging device is employed in the pre-secondary transfer charging section. However, even when the scorotron charging device is employed, a potential decrease at a low potential portion would occur to some extent. Accordingly, when simply employing a strong discharging operation, although a sufficient discharging effect for portions carrying a large amount of toner can be achieved, an electric potential at the portions carrying a small amount of toner are also lowered. On the contrary, when a discharging operation is weak, it is impossible to decrease a potential value of portions carrying a large amount of toner to a desired value, resulting in a difficulty of satisfying both of the above-mentioned factors at the same time.
With respect to a conventional case in which a discharging operation before secondary transfer is conducted once by employing a single discharging device, the present inventors measured the potentials of the toner layer after applying a discharging operation versus the potentials of the toner layer before applying a discharging operation, by setting a grid voltage of a single discharging device, disposed at a pre-secondary transfer charge eliminating section, at three levels of −150V, −100V and −50V. As a result, the present inventors confirms that it is impossible for the single discharging device to achieve the adjustment of a toner layer potential, which satisfies both of the appropriate discharging operations for the portions carrying a large amount of toner, such as an overlapped solid color image, etc., and the portions carrying a small amount of toner, such as a halftone color image, etc., resulting in an inability of obtaining a good image.